U.S. Pat. No. 4,686,437 to Langley et al. entitled Electromechanical Energy Conversion System describes a motor control system whereby a microprocessor or similar electronics can be used to control electric motors. The sensitive electronics are coupled to the motor via a power transistor switching stage wherein the transistors operate in on/off fashion to control the current flow to the motor windings in accord with control signals from the microprocessor. The microprocessor receives feedback from the motor indicating rotor position and other controllable parameters. The microprocessor may also receive command signals usable for servo control. The command signals and the feedback signals are compared to determine the frequency, amplitude and phase for the winding energization, and the rotor position feedback is used to determine the proper commutation of the winding excitation. The microprocessor, either alone or with other components, provides on/off control signals to the power stages which, in turn, supply current to the windings with the desired amplitude, frequency and phase commutated according to the rotor position. Most present day servo motor control systems utilize this basic control system.
U.S. Pat. Nos. 4,447,771 and 4,490,661 to Whited and Brown et al., respectively, disclose motor control systems in which digital electronics control power switching transistors via a pulse width modulator (PWM) so that the excitation current to the motor is a function of the pulse width. Both Whited and Brown et al. disclose techniques for controlling the phase angle of the excitation current relative to the rotor position to improve the torque efficiency of the system.
Most present day systems use pulse width modulation as disclosed by Whited and Brown et al. to convert the control signals from the sensitive electronics to the desired excitation currents for the motor windings. In such systems used to control large motors isolation is necessary between the sensitive electronic controller and the brute force power stages that drive the motor. Most present day systems employ optical coupling which provides a high degree of isolation in the present controller designs which operate at a PWM switching rate of about 2 KHz.
There is an ever present desire to provide smaller and less costly controllers and controllers with high overall system efficiency. The invention provides those advantages for any size motor. However, the advantages are more pronounced for large motors in the range of 10 to 150 horsepower and most pronounced for the very large motors in the 150 to 500 horsepower range. In the higher horsepower ranges inefficient use of components and inefficient overall system design directly translates into high controller costs and high usage costs.
One approach to smaller and less costly controllers is to increase the switching frequency of the PWM power stage. In theory at least, higher switching frequencies should make it possible to control the needed power for large motors using smaller and less expensive components. However, the problems associated with operating the power stages at higher switching frequencies have proven to be either insolvable or prohibitively costly both in space and money. High frequency noise can bleed off power from the system and seriously affect the system efficiency. Noise of the common mode variety (noise that goes up and down together on a parallel conductor pair), can be particularly difficult to detect and even more difficult to eliminate.